Object alignment, was: cygwin failures - assertion "!(addr & FLAGS)" failed:
Sun May 5 17:23:00 GMT 2002
On Sunday 05 May 2002 16:49, Billinghurst, David (CRTS) wrote:
> cygwin readers: This is about porting java to cygwin for gcc-3.2
> (and perhaps gcc-3.1.1 if the changes are small).
> So we need objects to be 8-byte aligned so that the low three bits
> of the address are 0? Are there any ways around this, as I don't
> think that this is the default on cygwin.
> I seem to recall that:
> - objects are only 4-byte aligned by default.
The binutils alignment parameter in coff-i386.c is set to 4-byte alignment.
4-byte alignment also is the default for gcc -Os, at least from gcc-3.1.
Cases have been produced where gcc-3.1 failed to give 16-byte alignment
even with -O2, unless -mpreferred-stack-boundary=4 was specified.
> - it is necessary to rebuild ld to ensure 8-byte alignment
not only ld, but all of binutils
> - there are performance benefits for 8-byte (or even 16-byte)
> alignment, but arguements against.
8-byte alignment is required for reasonable performance with 64-bit data.
16-byte alignment is required by <xmmintrin.h>, which uses aligned sse
instructions, and for reasonable performance with un-aligned sse instructions.
P4 does not share the 16-byte code alignment requirements for full
performance which P-II/P-III have.
I have been building and testing gcc-3.1 in cygwin with 16-byte alignments
specified, wherever I know how, with no ill effects, and I consider it a must
16-byte alignment is incompatible AFAIK with the libstdc++ of gcc-2.95.x, but
the C, f77, and objc compilers have no problem with it.
The primary argument I have seen for making 4-byte alignment the default for
-Os in gcc had to do with the 1MB stack per thread limitation being easily
exceeded. Future glibc should always have at least 8MB stack per thread. I
don't know the answers for the cygwin environment.
Another argument which has been made in the past is that MSVC couldn't be
counted upon for better than 4-byte alignments.
> There is much discussion over a long period in the cygwin archives,
> which I will obviously have to re-read in the next little while.
> -----Original Message-----
> From: Tom Tromey [mailto:firstname.lastname@example.org]
> Sent: Saturday, 4 May 2002 6:31
> To: Billinghurst, David (CRTS)
> Cc: email@example.com
> Subject: Re: cygwin failures - assertion "!(addr & FLAGS)" failed:
> >>>>> "David" == Billinghurst, David (CRTS)
> >>>>> <David.Billinghurst@riotinto.com> writes:
> David> At last a real bug in the cygwin libjava testsuite. Most of the
> David> compilation tests pass, but all (that I could find) executable
> David> tests fail with
> David> assertion "!(addr & FLAGS)" failed: file
> "/usr/local/src/gcc3.1/libjava/java/lang/natObject.cc", line 772
> My understanding is that we use the lowest 3 bits to keep some
> information about the thin locks. If this assertion fails, it means
> that we've found an object which isn't suitably aligned. I think this
> can mean one of two things:
> 1. There is a bug in the allocator (unlikely), or
> 2. The compiler isn't properly aligning an object which it lays out
> statically. For instance this could happen with some Class object
> A bug along these lines was fixed pretty recently:
> 2002-04-18 Bryce McKinlay <firstname.lastname@example.org>
> * class.c (make_class_data): Set DECL_ALIGN on static class data,
> for hash synchronization.
> * expr.c (java_expand_expr): Set DECL_ALIGN on static array objects.
> * decl.c (java_init_decl_processing): Don't set TYPE_ALIGN for
> If you already have this patch then I guess there's another such bug :-(
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